1. Field of the Invention
The present invention relates to a method for heating silicon carrier bodies in a deposition reactor by means of radiated heat. The invention also relates to a device for carrying out the method.
2. The Prior Art
Polycrystalline silicon rods, so-called polyrods, are produced by thermal decomposition of gaseous silicon compounds in a deposition reactor, elemental silicon being formed. In the deposition reactor there are heated carrier bodies, so-called slim rods, on which the silicon deposits They are normally also composed of silicon and grow in the course of the deposition to the desired polyrods. The slim rods are brought to the required deposition temperature by direct current passage. They have to be heated beforehand by means of a heat radiation system until the so-called firing temperature is reached at which the ohmic resistance with which they oppose the current flow when a voltage is applied has become sufficiently low. It is only then that further heating to the deposition temperature takes place by direct current passage. The polyrods produced are an important basic material for the production of high-purity silicon, for example for the production of silicon monocrystals. They must therefore meet very strict purity requirements.
In accordance with German Patent No. DE-28 54 707 C2, to fire the slim rods, a heating finger is lowered down from the reactor top to just above the deposition-reactor base with the reactor open. After the firing temperature is reached, the heating finger is retracted from the deposition reactor again and the opening of the deposition reactor is closed by means of a lid. It is particularly unfavourable that the deposition reactor has to be opened comparatively wide, under which circumstances oxygen can easily get in. In particular, there is the danger that particles from the surrounding room air or particles which originate from the heating finger itself get into the deposition reactor and are later incorporated in the polyrods as impurities. Before the deposition of silicon on the carrier bodies can be started, the deposition reactor has to be carefully flushed with inert gas in order to remove entrained oxygen again. Particles which had been entrained previously can, however, only be removed thereby to an inadequate extent.
Heating fingers which comprise a bundle of metal-clad electrical heating coils are used. They are surrounded by a cylindrical quartz sheath which is flushed with inert gas. Current is passed through the heating coils, so that they are heated to about 900.degree. C. and are able to bring the slim rods to firing temperature. Because of the high temperature loading and the presence of traces of oxygen, the heating coils, the metal cladding and the electrical contacts of the heating finger are, in particular, subject to continuous oxidative corrosion. This circumstance makes short maintenance cycles necessary, and during these the heating finger cannot be used. In addition, as a result of the continuing corrosion, particles are continuously formed which may contaminate the polyrods.
Further disadvantages result from the size of the heating finger. Since the heating finger has to extend down to near the base of the deposition reactor so that the slim rods can be heated to the firing temperature, the deposition reactor has to be accommodated in unusually high operating rooms. Only then is there enough room available for the heating finger above the open deposition reactor. In addition, the heating finger is relatively heavy as a result of its weight of about 200 kg and requires a lifting and lowering device which can move such loads reliably